1. Field of the Invention
The present invention relates generally to an observation optical system configured to present one combined and enlarged image by combining light fluxes from a plurality of display elements, and more particularly to an observation optical system suitable for an image display apparatus, such as a head mounted display (“HMD”).
2. Description of the Related Art
FIG. 13 of Japanese Patent Laid-Open No. (“JP”) 2009-115906 discloses a structure for an observation optical system configured to lead an image at a wide field angle to an exit pupil (eyeball), and this observation optical system includes two jointed assemblies each including three display elements and three optical units so as to widen a field angle and to enable original images of six display elements to be observed as a combined and enlarged image. Assume that a visual axis is defined by a center field-angle principal ray from the exit pupil of the observation optical system to an optical element (the two assemblies), where the center field-angle principal ray passes the center of the exit pupil of the observation optical system and goes to the center of the combined and enlarged image. Each assembly is an optical system element of the optical unit configured to enable an original image displayed on one display element to be observed as an enlarged image, and includes optical planes having only one symmetrical plane. The optical units are arranged in one assembly so that the symmetrical surfaces of the optical elements accord with each other. Now assume that a decentering section of the observation optical system is defined as a section that includes the symmetrical plane. Then, in JP 2009-115906, the decentering sections of the assemblies intersect each other, and a line of the intersection accords with the axis perpendicular to the visual axis that passes the center of the exit pupil. These two assemblies are arranged in a radial direction around the axis perpendicular to the visual axis.
FIG. 11 of JP 2010-266787 discloses an observation optical system configured to lead light fluxes from two image display elements to the exit pupil utilizing two optical elements each of which has a plurality of reflective surfaces and folds an optical path. The observation optical system presents a combined and enlarged image of the original images displayed on the two image display elements. Each of the two optical elements has such optical surfaces that there is only one symmetrical plane, and these two optical elements are arranged so that their symmetrical planes accord with each other. One optical unit leads the light flux from one display element to the exit pupil along the optical path via an incident surface, a transmitting/reflecting surface, a reflective surface, and the transmitting/reflecting surface of the one optical unit. The other optical unit leads the light flux from the other display element to the exit pupil along an optical path via an incident surface, a transmitting/reflecting surface, a reflective surface, and the transmitting/reflecting surface of the other optical unit and part of the one optical unit. These two optical units enable the light fluxes from the point having the same field angle displayed on the two display elements to overlap each other. Thereby, even when the eyeball moves, the image is not shielded and is displayed by maximizing the resolutions of the two image display elements.
According to JP 2009-115906, the image is not chipped when the pupil of the observer is located near the center of the exit pupil. However, the image is chipped as the eyeball moves to the periphery of the exit pupil. In addition, according to JP 2010-266787, the observation optical system becomes thicker on the visible axis than each optical element when the (visual) axis is defined by the center field-angle principal ray from the exit pupil of the observation optical system to the optical element, where the center field-angle principal ray passes the center of the exit pupil of the observation optical system and goes to the center of the combined and enlarged image.